The NAVSTAR Global Positioning System of the U.S. Air Force are made up of a plurality of Earth-orbiting, signal-transmitting satellites which transmit spread spectrum signals in accordance with a predetermined code. "NAVSTAR: Global Positioning System--Ten Years Later", B. W. Parkinson and S. W. Gilbert, Proc. IEEE, Vol. 71, October, 1983. In U.S. Pat. No. 4,797,677 to MacDoran et al, a system is disclosed for deriving pseudo range without knowledge of the code sequence of modulation carried by the signals. However, that system as devised was not concerned with the problem of tracking both the velocity and position of a moving object without knowledge of the code sequence of the satellites. For instance, it is highly desirable to be able to measure weather balloon velocity using an expendable, built-in global positioning system receiver and to be able to process signals at a ground station without knowledge of the pseudo random noise codes used by the GPS satellites and wherein the object to be tracked can include an inexpensive receiver which is capable of receiving signals from the satellites and transmitting either alone or in combination with other data to a central processing station. Specifically, it is desirable to be able to employ a very inexpensive reference oscillator on board the expendable receiver; or, in other words, an oscillator which is not necessarily accurate or highly stable. In this relation, it is important to be able to manage the effects of the receiver's instantaneous velocity vector projections onto the lines of sight to the individual GPS satellites that results in a Doppler shift to those particular GPS observations; and further to manage the effects of the kinematics of the receiver that results in a change of the spread spectrum signals induced by the change of geodetic location as the receiver moves between different locations.
In aforementioned U.S. Pat. No. 4,797,677, there were both explicit and implicit assumptions regarding the accuracy and stability of the reference oscillator utilized to accomplish the final down-conversion to baseband of the compressed spectrum obtained from non-linear operations on the spread spectrum signals; i.e., delay and multiply. The oscillators employed were relatively expensive, making them unsuitable for use in an airborne or other expendable receiver, such as, for example, for use as a part of an expendable airborne weather instrument. Moreover, the weight and power requirements of the more expensive oscillators made them entirely unsuitable for launch in a light-weight receiver.
It is therefore important to provide a system in which a light-weight, low cost oscillator may be incorporated into an expendable receiver, even though the accuracy and stability of the oscillator is on the order of 0.001% (10 PPM). This implies that at the reference frequency of 10.2298 MHz the actual frequency being generated is in error by approximately 100 Hz. The baseband signal bandwidth that contain the observations to be eventually processed into velocity and positioning information are essentially contained within .+-.27 Hz for a static Earth-based observer exploiting the P channel, which has a chipping code frequency of 10.23 MHz. However, where the mission is to track a moving object or receiver which may be moving at speeds of up to 200 m/sec the spectral lines extracted from the delay and multiple operation in the receivers or spectral compressor, may be at many possible spectral positions. Where the reference frequency may be off as much as 100 Hz, the spectral line ensemble, resulting from each satellite in view, may shift either to higher or lower frequencies by as much as 100 Hz. A further consideration is the shift in spectral line positions, for example, a 200 m/s motion of the receiver directly toward or away from a particular satellite would cause up to 6.8 Hz of additional Doppler shift which would be imposed onto a line of sight to the satellite. A still further consideration is the distance of the receiver away from the processing station. For example, for a P channel chipping frequency of 10.23 MHz for the GPS, the sensitivity to positional changes may be as much as 5 milliHertz per km and which for a 200 km separation may cause the spectral lines to shift by 1.0 Hz.
From the foregoing, it is possible to establish the rationale for determining the down conversion frequency to be used in the GPS receiver; namely, a crystal oscillator offset at 10 PPM at nominal 10 MHz:100 Hz; nominal Doppler shifts along GPS/receiver path .+-.27 Hz; receiver wind-induced Doppler shift (&lt;200 m/sec. ); 6.8 Hz; and receiver positional sensitivity (&lt;200 km range ):1.0 Hz. The dominant effects on the chipping frequency spectral line position are from the 10 MHz crystal oscillator in the receiver followed by the combined Doppler effects from the GPS satellites and wind velocity, and the least influential effect is from the actual position of the receiver. The sum of the worst case combination of these tolerances is 134.0 Hz. A further requirement is that the spectral lines remain on the same side of the zero frequency for ease of processing. Assuming that the center frequency of the compressed baseband is placed at 200 Hz, the negative tolerance of 134 Hz makes it necessary to process a band extending from 66 Hz to 334 Hz. An oscillator of 10 PPM accuracy can be specified no more closely than 100 Hz and accordingly the specification of the frequency should be 10.2298 MHz which will result in placing the spectral lines at a center frequency of 200 Hz, assuming that the oscillator has no offset.